The characteristics of glasses and fibres based on zirconium fluoride are reviewed. The fundamental limits to attenuation due to intrinsic processes in these materials are surveyed, leading to a predicted minimum loss approaching 0.01 dB/km at the likely operating wavelength of 2.55 μm. Current limitations due to extrinsic scattering and absorption are analysed, showing that most of the excess loss comes from wavelength-independent scattering due to small bubbles, crystallites and other small particles. These extrinsic mechanisms limit the attenuation to around 1dB/km in the best current fibres. Design criteria for single mode fibres are also reviewed, taking into account dispersion and mode field effects to optimise microbending and splicing characteristics.